1. Field of the Invention:
The invention relates in general to elevator systems, and more specifically to elevator systems of the traction type.
2. Description of the Prior Art:
In elevator systems of the traction type, the rating of the traction drive machine connected to rotate the drive or traction sheave is determined by the traction efficiency of the arrangement. Traction efficiency is responsive to the ratio of the rope and/or sheave life to the required tractive effort. In other words, the traction drive machine must satisfy the functional requirements of elevator loading and traction within limitations imposed by the wear rates of ropes and sheave to obtain a satisfactory operating life.
In the prior art, geared traction elevator drives usually use a single or half wrap hoist cable and sheave arrangement, and the cable grooves are undercut to wedge or pinch the cables or ropes to increase the tractive effort. This arrangement, however, hastens cable and sheave wear and increases the operating pressure in the groove. To reduce the groove pressure, a larger diameter sheave and/or a larger number of cables or ropes are required. Increasing the diameter of the drive sheave increases the torque arm and requires a drive with a higher rating.
In the prior art, gearless traction elevator drives usually use the double or full wrap hoist cable and sheave arrangement in order to provide the required tractive effort. The grooves are not undercut, but the additional bends in the hoist cables deleteriously affects their life and it increases the loading on the drive sheave, compared with the single wrap. Also, a larger sheave diameter is usually necessary in order to increase the radius of curvature in the rope as it wraps around the drive and secondary sheaves, requiring a drive machine with a higher rating.
The prior art has taken many different approaches in attempting to increase the traction efficiency. For example, U.S. Pat. No. 3,838,752, assigned to the same assignee as the present application, discloses the use of a synthetic lubricant which allows the cables to be adequately lubricated without an offsetting decrease in tractive effort, as the coefficient of friction between the cables and cable grooves provided by the lubricant is high enough to prevent slippage. U.S. Pat. No. 3,279,762 discloses lining the cable grooves of an elevator drive sheave with a pliable or elastomeric element which is "treaded" to provide voids which trap any lubricant which may be exuded from the cables. The elastomeric grooves, while reducing wear, in general have very low allowable contact pressures and the traction capability of the sheave is thus deleteriously affected. U.S. Pat. No. 1,438,674 discloses a mechanical arrangement for forcing the hoist cables of an elevator system tightly into the groove of the drive sheave to increase the traction between the cable and sheave groove. U.S. Pat. No. 432,701 discloses lining the rim of a free running pulley, i.e., not a driven sheave, with alternate blocks of rubber and wood. The wood is grooved to guide a cable and the rubber contacts the cable to reduce slippage of the cable as it passes around the pulley.
It would be desirable to provide a new and improved arrangement for increasing the traction efficiency of a traction elevator system, without compromising the load and traction capability of the drive sheave.